Hydrochloric acid is an important mineral acid which is corrosive and hazardous since it causes severe burns to human tissue. Hydrochloric acid also reacts with most metals to form explosive hydrogen gas. Concentrated hydrochloric acid is transported and stored in rubber-lined or polyester reinforced plastic tanks. Processes which use the aqueous acid are commonly carried out in glass-lined steel equipment involving obvious limitations and problems. Metals and alloys for handling hydrochloric acid are, therefore, very desirable, and suitable candidates are judged to be those with a corrosion rate under about 20 mils per year (MPY) when exposed to uncontaminated acid. In practice contamination is not uncommon and can cause catastrophic equipment failures. For example, acid recovered from the manufacture of fluorocarbons may contain over 0.5% hydrogen fluoride yet as little as 10 parts per million or less of fluorides may severely damage equipment made of glass-lined steel or the refractory metals, such as titanium, zirconium, columbium and tantalum.
Also, the presence of ferric ion in hydrochloric acid can drastically increase the corrosion of unalloyed zirconium, copper alloys, copper-nickel and nickel-copper alloys, as well as the nickel-molybdenum alloys known as Hastelloy B and B-2. Cupric ion behave somewhat similarly to ferric ion with all of the same metals and alloys. Aeration or contamination by other oxidizing substances will quickly result in failure of the Hastelloy B or B-2 alloys which otherwise have corrosion rates under 20 MPY in all concentrations of acid and temperatures up to the atmospheric boiling point in nonaerated acid and in the absence of oxidizing agents. Hydrochloric acid can also become contaminated with organic solvents when recovered as a by-product of a chlorination process. Even a few parts per million of organic contaminants can over a period of time, destroy rubber linings and certain plastics and elastomers.
Cast iron, containing over 14.3% silicon plus up to 3% molybdenum and 4% to 5% chromium, is suitable for all concentrations of hydrochloric acid to about 125.degree. F., but is hard and extremely brittle. Nickel alloys containing about 9% silicon and 3% copper are slightly less brittle but still quite hard and suitable only for up to about 15% acid strengths at room temperature.
The commonly used austenitic stainless steels such as types 304 and 316, are not resistant to hydrochloric acid at any concentration and temperature. Higher nickel and molybdenum contents and, to a lesser extent, copper, in non-standard modified stainless steels impart some resistance to dilute acid, but pitting, local attack and stress corrosion cracking can still result. The commercial alloys known as Inconel 825 and Hastelloy G contain over 40% nickel and have useful resistance to all concentrations of hydrochloric acid below about 100.degree. F. Inconel 625 contains about 62% nickel and the higher nickel has very good resistance to concentrated reagent grade acid at ambient temperatures. These alloys also contain chromium and molybdenum acid.
The most corrosion resistant of the nickel-base alloys to hydrochloric acid are Hastelloy B-2 (Ni-28Mo) and Hastelloy C-276 (Ni-16Cr-16Mo-3W). Alloy B-2 is excellent in the absence of oxidizing agents but quite expensive. Alloy C-276 shows less than 5 MPY attack in all concentrations of the acid at room temperature and less than about 20 MPY in all concentrations up to about 120.degree. F., but it is also quite expensive.
Zirconium, titanium, tantalum, columbium, molybdenum and, to a considerably lesser extent, gold, silver and platinum, have all found some use in hydrochloric acid environments, but all are very expensive and present many fabrication and other problems. Carbon steels and standard ferritic stainless steels have no resistance to hydrochloric acid, while a few of the high-purity, proprietary, high-molybdenum stainless steels are useful for acid concentrations only up to about 1.5%. No duplex stainless steel has yet been developed with any significant usefulness in hydrochloric acid.
Thus there has remained a very keen interest in developing iron-base modified stainless steels of much lower nickel contents than the current nickel-base and related alloys but of useful resistance to hydrochloric acid as well as to local corrosion and stress corrosion cracking.
Molybdenum is the most effective addition element used in alloys to develop resistant to hydrochloric acid. Molybdenum, chromium and nitrogen all build resistance to local corrosion and stress corrosion cracking. Since molybdenum and chromium are both strong ferrite forming elements, use of large amounts of these elements together necessitate high nickel levels in order to maintain the required single phase austenitic, or face-centered-cubic, matrix crystal structure.
Spitz, U.S. Pat. No. 2,633,420, discloses alloys of more than 6% but not more than 20% each of chromium, nickel, copper and molybdenum, the sum being less than 50% and the remainder being substantially all iron. In the presence of these amounts of chromium and molybdenum such alloys (of less than 20% nickel content) will have a maximum of about 4.5% copper since that is all that can be retained in solid solution in a stable state. Therefore, all alloys of the composition ranges of elements specified by the '420 patent will contain some copper-rich precipitates, which render these alloys highly corrodible by hydrochloric acid. Alloys composed of most of the possible element ranges of proportions will contain other additional phases such as sigma, laves, alpha and carbides. Thus all alloYs of the '420 patent would be considered by modern standards to have very limited and poor corrosion resistance and of no practical usefulness to any concentrations of hydrochloric acid at any temperatures.
Alloys that resist general attack by hydrochloric acid rather well but are still susceptible to local corrosion and stress corrosion cracking remain impractical for hydrochloric acid service. Many alloys withstand general surface attack in some HCl acid strengths (usually dilute) but still fail from local corrosion or stress corrosion cracking, thought to be the result of corrosion products formed, particularly FeCl.sub.3. On the other hand, alloys of modest or low nickel contents have been developed that resist chloride local attack and stress corrosion cracking very well but are still not especially resistant to hydrochloric acid. Examples of such alloys are disclosed in Hatfield, U.S. Pat. No. 2,402,814, Rassomme et al, U.S. Pat. No. 4,421,557, Baumel, U.S. Pat. No. 3,726,668, Abo et al, U.S. Pat. No. 4,172,716 and the Japanese patent J57016-153, all of which disclose alloys containing less than 6%-7% molybdenum. There have also been many patents disclosing duplex stainless steel which contain up to about 6% Mo, such as Yamaguchi et al, U.S. Pat. No. 4,141,762. But, as noted above, duplex stainless steels do not resist hydrochloric acid well.
Kudo et al, U.S. Pat. No. 4,400,349, claims alloys of 20-60% Ni, 15-35% Cr, 0-12 Mo, 0-24% Cr, 3-20% Mn plus iron and other elements. The exemplary alloys all contain between 20.7% and 59.6% Ni, while those having a molybdenum content range of 7-9% also contain 16.2% to 29.2% Cr, necessitating the inclusion of a combined nickel plus manganese content of 30.3% to 50.1% in order to maintain structural stability.
Fleischmann, U.S. Pat. No. 2,398,702, claims copper-free heat resistant alloys of up to 0.15% carbon, 4-8% Mo, 12-20% Cr and nickel in an amount such as to render the alloys austenitic alloys included in that patent, such as the alloy known as 16-25-6, contain precipitated carbides and are of generally poor corrosion resistance.
Japanese patent J5 7171-651 claims alloys of 4-8% Mo, 18-25% Cr, 20-30% Ni, 0.3-3% Cu and lanthanum plus cerium in addition to iron and other elements. The exemplary alloys are essentially of the 25% Ni, 20% Cr, 6% Mo type.
Liljas et al, U.S. Pat. No. 4,078,920, claims alloys of 17-25% Cr, 15-21% Ni, 6-10% Mo, up to 2% Cu, up to 1% Mn and the remainder iron plus carbon, nitrogen, silicon and impurities. The commercial alloy covered by this patent is known as 254SMo and nominally contains 18% Ni, 20% Cr, 6.1% Mo, 0.8% Cu, 0.20% Ni, 0.5% Mn, and the balance essentially iron. Liljas specifies a maximum of 2% Mn and states that a maximum of 1% Mn is preferred. My patent U.S. Pat. No. 4,818,483, discloses alloys which are an improvement over the alloYs of Liljas and contain ranges of elements otherwise similar to the alloys of Liljas except for a 3-8% Mn content. Alloys of both '920 and '483 possess good resistance to many chlorides but not very good resistance to hydrochloric acid.
Thus it has remained desirable to develop alloys of good mechanical properties and good resistance to hydrochloric acid but containing a nickel content of 20% or lower instead of in the 40-60% Ni range of prior art alloys.